Tag Archives: PCB Manufacture

PCB fabricators in the UK are facing the challenges of being able to make the PCB’s that the designers need to accommodate these modern packages. This is no easy challenge, fabricators based in Asia and other European countries are providing their customers with advanced capabilities that not long ago were not possible. Not only do these fabricators have the capability, but the manufacturing costs cannot be matched equivalent fabricators in the UK.

Electronics are advancing at an ever increasing rate and this rate has no sign of abating. Integrated circuits are being developed all the time with increasing speeds, functionality and higher densities than were available before.
This gives engineers the components they need to create products with more functionality in a much smaller form factor than has been possible before. No other market shows more evidence of this than the smart phone and tablet market. These supercomputers in our pockets were inconceivable in such a compact size fifteen years ago.

Chip packages come with far more, smaller connections, whether they’re balls, pads or pins in a much more compact package. In for UK fabricators to win business to serve this insatiable demand for smaller and better electronics, they need to be able make the boards that are being designed today and be looking forward to the designs needed in the years to come. It doesn’t seem that long ago that a track and gap of less than 0.2mm would be the exception rather than the norm, but modern electronics has ended this forever.

It seems however that many UK fabricators are struggling to realise this and are not investing enough to keep up with the rate of change.

The high density PCB packages of today are forcing designers to use track and gaps of less than 0.1mm or copper filed via in pad, or via sizes of 0.1mm just to be able to route connections out to the rest of the circuit. BGA’s aren’t the only issue, dual row and other leadless packages where the pins are 0.5mm apart or less are forcing designers to make tough choices about where they place the compromise in their designs.

BGA’s aren’t the only issue, dual row and other leadless packages where the pins are 0.5mm apart or less are forcing designers to make tough choices about where they place the compromise in their designs.

There aren’t many PCB fabricators left in the UK and only a handful are able to compete with the capabilities that offshore companies are able to give. The reality is that there is a tough job ahead to catch up and keep up and many PCB fabricators haven’t advanced their capabilities in over 10 years. This is an age in technology terms. The UK cannot possibly compete on cost with offshore manufacturing, but if they can’t do at least the same or better, there is no way to compete with the offshore fabricators.

Making reliable electronic assemblies is about making a good solder joint isn’t it?

So what’s the problem, plop down some paste in the right area place the component carefully down, apply heat and the solder melts. Hey presto!

Yes, if only it was that easy. The Jedec thermal profile is well known and was mentioned in a previous issue???? The problem is every part of a PCB will have a different thermal profile as will every component and joint. In the ideal world the flux needs to activate at the same time and the solder melt at the same time, but this is never going to happen.

The result of this is an electronic assembly can suffer from dry joints in places and others voids, head in pad or possibly tombstoning. It’s quite a juggling act that needs to be performed. Unfortunately it’s mainly trial an error, but knowing what to consider before going to assembly can help avoid some of these nasty niggly errors.

It’s more difficult to uniformly heat a PCB to the correct Jedec profile. Thermal shadows and hotspots in different parts of the PCB can create problems in achieving a good result. Using a longer thermal ramp up and dwell time can often help but getting heat under leadless components and BGA’s will always create difficulties.

Dry joints and voids under these components will not be immediately obvious with only x-ray inspection able to reveal any issues. Many BGA’s have hundreds of balls on them with every singe one needing to be perfectly soldered in order to operate properly and reliably.

Vapour phase reflow is becoming more popular in reflowing modern high density PCB’s as this gives a far more uniform and reliable thermal profile across the PCB being assembled, even for joints under the component such as BGA’s. It tends to be a slower way of reflowing boards, so it isn’t used as much in higher quantities, but the process gives good results across the board.

So now we know how to make a good solder joint that’s it right? The board can be put into service and will work for years – problem solved?

No.

Many PCB’s are put into service in environments that are harsh, with high levels of moisture, dust, or some kind of chemical pollution. Any one of these will affect the long term operation of the product by either forcing the board to overheat or causing problems in the operation of the circuit.

Using underfills for BGA’s and a conformal coating can help protect a circuit from these effects, but a maintenance schedule could still be needed to clean the boards.

An underfill is a liquid that fills in under a BGA or other leadless component to protect the joints and give extra mechanical rigidity.

Conformal coating are sprayed over a PCB to protect the board from moisture and other contaminants. There are many types to use and choosing the right one to protect against the contaminants it’s expected to encounter will need careful consideration.

Both of these processes should only be applied to a tested working board. If these are to be used on a cleaned board, the board must be really clean as any residue flux is likely to corrode any joints or copper over time (such is the nature of no clean fluxes). Once a conformal coating has been applied, there’s little that can be changed on a board, so it’s make or break. If all this works, your PCB will work in some harsh environments for a good time without trouble.

When I got my first Android phone it was suggested I look up an app called Electrodroid. After installing it, I was instantly impressed. Electrodroid is only available on Android and is the app to have for any and every kind of electronics engineer and technician. As shown above there are four tabs to choose a vast array of useful information that’s available on the go from your device.

Calculators – has a range of different calculators to help with everything from resistor codes to filters and PCB trace width calculation. All of this in one place.

The next tab has a whole list of

Pin-Outs – think of a connector used in everyday electronics – it’s probably here. On top of this are pin outs for Raspberry Pi, Arduino’s and Beaglebone boards also.

The next tab is a Resource table for component selection, wiring size tables and chip databases.

The amount information available to the Electrodroid user is massive and it’s accurate, quality information that can be relied on.

Electrodroid is also expandable, with a number of Plugins that are designed to work with it.

It’s when the plugin’s for Electrodroid are used that the true power of this little app becomes apparent. The plugins allow us to add extra functionality to:

Access a complete parts database.

PIC Microcontroller Database.

Atmel Micro Database.

Circuit Simulation.

That’s right – circuit simulation on a mobile device. While this may be hard to use on a phone, a 7 or 10 inch tablet offers a useable environment for this feature.

The Circuit Simulator which is offered by Everycircuit and has in app costs up to £12.

As with many of these apps the basic version was free, with a paid version offering more features. The cost of the paid version only around £2 the investment is a no-brainer and easily worth more as the plugin features that can go with Electrodroid are free to use (apart from the simulator).

Access to an integrated electronics design assistant as powerful as this – How much can it change how engineer’s and technician’s work?

Multilayer PCB’s are being used in almost every electronic device that can be thought of. This is especially true in higher density electronic devices where components are crammed into the smallest space possible. The reason for this? It enables the engineer to track out the circuit in a tight area while providing functionality like impedance control.

Multilayer PCB’s also provide excellent thermal conductivity to be able to get heat away from components without the need of heat sinks.

This kind of approach is bound to have an effect on the way PCB’s are assembled and re-worked. Soldering parts down to a board can be hard enough, but getting them off again without damaging the board can be much harder.

Assembly and Reflow:

Applying the paste and placing components onto a multilayer PCB happens in the same way as a single sided PCB. The problems come during reflow. Imagine the PCB that’s being worked on now going through a big heater and being warmed up to a thermal profile like this one:

The area’s of copper that are larger are going to sap the heat away faster and will therefore take longer to heat up than the area’s where there is little excess copper. So all the different parts of the circuit are warming up at different rates. This could potentially lead to solder re-flow taking place at different times. This could lead to components tombstoning, reflow not taking place underneath IC’s with power pads and dry joints. Basically – a PCB engineer’s nightmare.

Compensating for this is usually done by increasing the dwell time (referred to as the soaking zone in the chart) the board is subjected to. This ensures that every part of the PCB is at the right temperature for the fluxes to start working and will reach the desired peak temperature, with all the solder joints forming as they should.

Another reflow process used is the vapour phase reflow process, where heat transfer liquid gasses are used to heat every part of the PCB and the components uniformly. The issue with the vapour phase process is that it’s not a mass production process and is often slower than using IR reflow ovens. The vapour phase process can damage some sensitive components, so make sure evert component is suitable.

Rework & Repair:

In order to carry our PCB repairs, two things are needed:

1)The right tools

2) A well trained / experienced operator

A good soldering iron like a Metcal system with interchangeable tips and a talon system are essential to carry out good re-work. The heat is used efficiently and the tips transfer the heat well. A good rule of thumb is to use the largest tip possible, particularly in the removal process. A talon system allows the operator to use different sized tips to ‘pinch’ the component on each side and remove very easily with no PCB damage incurred.

PCB damage during component removal is common, but with modern small pad sizes, not as easy to repair as ten or more years ago.So knowing how to get enough heat into the part without damaging it or he PCB is necessary.

When removing a part from the board, any kind of force will damage the board or the pads. Using a pair of tweezers or other removal tool the part should lift off with no effort. Taking time here and being patient will ensure a successful re-work.

Now for the tricky bit – what about if we’re trying to rework something without legs, particularly a component with a power pad on the bottom?

Getting heat into a multilayer board can be a real issue for re-work, the greater the layers, the more of a problem it is. This is because the copper in each layers saps the heat away from the part. We mustn’t be too critical, after all that what we want it to do! But when we want to re-work, this definitely bites us where it hurts. With a production board going into the field using a quality re-work station is absolutely the best way.

If the boards are prototypes and a fast re-work is needed, it’s useful to be able to do this in house. Some good tips for making this easier are:

1) A quality, temperature controlled hot air pencil is often enough.

2) If a pencil alone is not enough use a pre heater under the board to heat the re-work area.

3) Use a large iron tip to help pre-heat the specific component.

The basic thing to bear in mind is that more copper means more energy to heat a board or component. Designs are now in this catch 22 – like it or not!

Multilayer PCB’s are layers of copper area’s etched to their patters to form a single circuit that all connects together, just like in the design. Easy eh?

We wish!

When we scratch away at the process behind the design we lean that maybe all is not what we might think. Defining the stackup before routing even starts is essential. This is because the stackup will define where the designer can route, where ground and power planes are located and where any controlled impedances will be routed too.

Time spent at this stage isn’t wasted, it’s well spent as this can save time later in tearing up chunks of design work and starting again.

Being realistic about the number of layers will also be beneficial at this stage, this means balancing (as always) cost against technical performance.

Discussing the requirements with the fabricator at this stage means that this stage of the build is designed and we don’t have to mess with it later.

Copper balance is a factor to be considered that few outside PCB design and fabrication know about – yet with multilayer boards it can have a profound effect. The effect of bad copper balance is a warped board and is caused if a board is designed with some areas of highly dense copper features where on other parts it’s less dense. Planes which over lap across the layers is often enough to cure this, either that or filling in between routed areas with ground pour. If doing this, make sure that no creepage distances, or other circuit features are compromised.

Drill holes and vias seem like a simple affair – on the whole they are. But when a plated hole is defined in your design – is this the size of the hole drilled or after plating? Normally this isn’t an issue, but if using custom assemblies with fine wires, problems can arise where the wire doesn’t fit in the hole as expected.

Pouring ground shapes – the design system needs to keep the features it creates large enough to be successfully etched. Tiny copper features can come adrift and cause circuit havoc. So as usual check what the fabricator can make and set the pour tool and DRC to filter any tiny features out – or a re-design is likely to follow sooner than expected.

Layer order. Every fabricator has their own default definition of layer orders.

Don’t assume that they’re all the same – make sure that each of the layers in the design are well labelled and the order of the layers is clear.

The effect of incorrect layer order may be a semi functioning circuit, but with controlled impedances referenced to power planes rather than ground, there may be some unusual and hard to find problems occurring.

As with everything in the PCB world, every part of the process has rules, processes and limitations. Knowing what they are and working with these processes is going to lead to better made designs.

It’s not greatly known that Intercept, the company behind the Pantheon PCB / Hybrid design software had released a controlled impedance calculator for mobile and tablet.

The app is available for either Android or Apple platforms and the first impression is a clean, uncluttered look. There are seven basic options and each one brings up the required variables. The power of this app is in it’s ease of use, the user prompted for the required values.

Where there is a calculator next to a value is where the user can leave the value blank. Input the other variables, press on the one remaining (which is shown in red) and there you go – your required value appears.

After diving into the other functions in the app the operation is much the same, the amount of variables change depending on what’s being calculated, but the user only needs to have a basic understanding to operate this and get a sensible answer.

The only thing I found was that the app defaults to mils dimensions automatically every time the app is used. I tend to work in metric these days, but it’s not a big deal, just mildly irritating.

I haven’t compared this to Polar or other high end software, but I have compared it against a few known and is pretty accurate. This is a free app that’s meant to be a guide and not replace the likes of Polar. On that basis it’s a very useful tool for any PCB engineer’s toolkit.

Todays world of high speed, high pin count high power density electronics means designing multilayer PCB’s is almost essential for any design. Multilayer boards enable a designer to be able to integrate controlled impedances for high speed and RF circuits with power circuits easily and efficiently without taking up large amounts of space and eliminating the need to use jumper links to make connections.

But as engineer’s knowing how our multilayer boards are built is key to ensuring design success. Knowing the materials that will be used at the fabricator, so the right cores and pre-pregs are used is are particularly important for impedance control. Allowing enough time and effort to work out the number of layers required and their stackup will be time well spent. While this can be changed during the design, it may mean that other design elements will be affected and time wasted in re- design to account for this. So it needs to be planned well.

So, how does a PCB Designer work all of this out?

Layers – Deciding on grounding and power strategies will give us the best guide on the layers we need. Ask these questions:

1)How many ground planes needed? Are there different ground on different planes or are they split?

2)How many power rails are there? Is a layer being allowed for each power rail or are we splitting them? Will they be split into area above each other to stop ground bounce?

3)What and how many signals are there to route? Will there need to be signals routed on inner layers and will these need to be protected from the power layers?

4)Just a plain through hole board or are blind / buried vias being employed?

Plane Arrangement:

The plane arrangement is typically number of planes one on top of the other on different layers. They will be a mix of power and ground nets. This is an acceptable way to arrange power and ground planes.

However in order to minimise ground bounce in a circuit, the power and ground planes could follow the same shaped profile on each layer, this reduces the coupling between power and ground. It could be any shape, but would follow the same profile on each layer for power and ground.

Once the basis for the multilayer board is decided, talked over with the fabricator, the placement and routing can begin in earnest. Placement is one of the key elements in achieving design success – get it right and you’re a big step towards making routing much easier. The big question here is – will there be components on both sides of the board?

Routing is next and is one of the elements of board design that can go wrong if not carried through methodically:

1) Ground comes first – always. If the ground connection is being made to an inner layer on a via, make the track thick – around 0.5mm and keep the via close to the component, particularly on decoupling components. For larger components make multiple ground connections, to give a low impedance connection.

2) Power – don’t underestimate this, get it right to avoid issues later, think about the power path. For example make sure the power connection flows from the plane to decoupling to component in that order. Not the other way around.

3) Controlled Impedances / Sensitive Signals – These signals are next in priority. The signals’ sensitivity will dictate it’s priority.

4) Everything else in it’s own natural order.

Following these steps should ensure the right circuit elements are routed with the right priority to achieve design success!

Finally, once all the connections have been made check the copper balance. Area’s of different copper density could cause the PCB to warp during fabrication. Filling in the blank area’s with ground pour where this won’t affect function or safety will ensure a uniform copper density and flat boards.

This of course is a list of the basic elements of multilayer design, but if the designer thinks along these lines and how the circuit will work before routing a single track – the path to success is less troublesome.